1. Field of the Invention
The invention relates to a method of manufacturing a thin film magnetic head having at least an inductive magnetic transducer.
2. Description of the Related Art
Recently, an improvement in performance of a thin film magnetic head has been sought in accordance with an increase in a surface recording density of a hard disk device. A composite thin film magnetic head is widely used as the thin film magnetic head. The composite thin film magnetic head has a laminated structure comprising a recording head having an inductive magnetic transducer for writing and a reproducing head having a magnetoresistive (hereinafter referred to as MR) element for reading.
MR elements include an AMR element utilizing an anisotropic magnetoresistive (AMR) effect and a GMR element utilizing a giant magnetoresistive (GMR) effect. The reproducing head using the AMR element is called an AMR head or simply an MR head, and the reproducing head using the GMR element is called a GMR head. The AMR head is used as the reproducing head whose surface recording density exceeds 1 gigabit per square inch, and the GMR head is used as the reproducing head whose surface recording density exceeds 3 gigabits per square inch.
The improvement in the performance of the recording head is also sought in accordance with such an improvement in the performance of the reproducing head. Factors for determining the performance of the recording head include a throat height (TH). This throat height means a length (height) of a magnetic pole portion from an air bearing surface to an edge of an insulating layer for electrically isolating thin film coils for generating a magnetic flux. The air bearing surface means the surface of the thin film magnetic head facing a magnetic recording medium and is sometimes called a track surface.
A reduction in the above-mentioned throat height is desired for the improvement in the performance of the recording head. This throat height is also controlled in accordance with an amount of polishing the air bearing surface.
An increase in a recording density of the performance of the recording head requires the increase in a track density of the magnetic recording medium. For this purpose, it is necessary to realize the recording head having a narrow track structure. In this structure, a bottom pole and a top pole, which are formed on the bottom and top of a write gap sandwiched between the bottom pole and the top pole, have a narrow width of from a few microns to the submicron order on the air bearing surface. Semiconductor processing technology is used in order to achieve this structure.
One example of a method of manufacturing the composite thin film magnetic head will be now described as one example of a conventional method of manufacturing the thin film magnetic head with reference to FIGS. 36 to 38. FIGS. 36 to 38 show a cross section of the thin film magnetic head perpendicular to the air bearing surface.
In this manufacturing method, first, as shown in FIG. 36, an insulating layer 102 made of alumina (Al.sub.2 O.sub.3), for example, is deposited with a thickness of about 5 .mu.m to 10 .mu.m on a substrate 101 made of altic (Al.sub.2 O.sub.3 and TiC), for example. Then, a lower shield layer 103 for the reproducing head is formed on the insulating layer 102. Then, alumina, for example, is sputter deposited with a thickness of 100 nm to 200 nm on the lower shield layer 103, whereby a shield gap film 104 is formed. Then, an MR film 105 for constituting the MR element for reproducing is formed with a thickness of a few tens of nanometers on the shield gap film 104, and the MR film 105 is patterned into a desired shape by high-accuracy photolithography. Then, a lead layer (not shown) for functioning as a lead electrode layer electrically connected to the MR film 105 is formed on both sides of the MR film 105. Then, a shield gap film 106 is formed on the lead layer, the shield gap film 104 and the MR film 105, whereby the MR film 105 is buried in the shield gap films 104 and 106. Then, an upper shield-cum-bottom pole (hereinafter referred to as a bottom pole) 107 made of a magnetic material for use in both of the reproducing head and the recording head, e.g., permalloy (NiFe) is formed on the shield gap film 106.
Then, as shown in FIG. 37, a write gap layer 108 made of an insulating film, e.g., an alumina film is formed on the bottom pole 107, and a photoresist layer 109 is formed into a predetermined pattern on the write gap layer 108 by the high-accuracy photolithography. Then, first-layer thin film coils 110 made of copper (Cu), for example, for an inductive recording head are formed on the photoresist layer 109 by plating method, for example. Then, a photoresist layer 111 is formed into a predetermined pattern by the high-accuracy photolithography so that the photoresist layer 109 and the coils 110 may be coated with the photoresist layer 111. Then, heat treatment takes place at a temperature of 250.degree. C., for example, in order to flatten the coils 110 and provide insulation among the coils 110. Then, second-layer thin film coils 112 made of copper, for example, are formed on the photoresist layer 111 by the plating, for example. Then, a photoresist layer 113 is formed into a predetermined pattern on the photoresist layer 111 and the coils 112 by the high-accuracy photolithography. Then, the heat treatment takes place at a temperature of 250.degree. C., for example, in order to flatten the coils 112 and provide insulation among the coils 112.
Then, as shown in FIG. 38, the write gap layer 108 is partially etched at the rear of the coils 110 and 112 (on the right side in FIG. 38) in order to form a magnetic path, whereby an opening 108a is formed. Then, an upper yoke-cum-top pole (hereinafter referred to as a top pole) 114 made of the magnetic material for the recording head, e.g., permalloy is selectively formed on the write gap layer 108 and the photoresist layers 109, 111 and 113. The top pole 114 is in contact with and magnetically coupled to the bottom pole 107 in the above-mentioned opening 108a. Then, the top pole 114 is used as a mask to etch the write gap layer 108 and the bottom pole 107 by about 0.5 .mu.m by means of ion milling. Then, an overcoat layer 115 made of alumina, for example, is formed on the top pole 114. Finally, a slider is machined, whereby a track surface (air bearing surface) 120 of the recording head and the reproducing head is formed. As a result, the thin film magnetic head is completed.
FIGS. 39 to 41 show the structure of the completed thin film magnetic head. FIG. 39 shows a cross section of the thin film magnetic head perpendicular to the air bearing surface 120. FIG. 40 shows an enlarged cross section parallel to the air bearing surface 120 in the magnetic pole portion. FIG. 41 shows a plan view. FIGS. 36 to 39 correspond to a cross section taken along line A-A' of FIG. 41. The overcoat layer 115 is not shown in FIGS. 39 to 41.
For the improvement in the performance of the thin film magnetic head, it is important to precisely form the throat height TH, an apex angle .theta., a pole width P2W and a pole length P2L shown in FIGS. 39 and 40. The apex angle .theta. means the angle between a straight line connecting corners of side surfaces of the photoresist layers 109, 111 and 113 close to the track surface and an upper surface of the top pole 114. The pole width P2W defines a write track width on the recording medium. The pole length P2L represents the thickness of the magnetic pole. In FIGS. 39 and 40, a `TH0 position` means the edge of the photoresist layer 109 that is the insulating layer for electrically isolating the thin film coils 110 and 112, close to the track surface. The TH0 position represents a reference position 0 of the throat height TH.
As shown in FIG. 40, the structure, in which the respective side walls of parts of the top pole 114, the write gap layer 108 and the bottom pole 107 are vertically formed in self-alignment, is called a trim structure. This trim structure allows a prevention of the increase in an effective track width resulting from a spread of the magnetic flux generated during writing on the narrow track. As shown in FIG. 40, a lead layer 121 for functioning as the lead electrode layer electrically connected to the MR film 105 is formed on both sides of the MR film 105. The lead layer 121 is not shown in FIGS. 36 to 39 and 41.
FIG. 42 shows a plan structure of the top pole 114. As shown in this drawing, the top pole 114 has a yoke portion 114a occupying most of the top pole 114, and a pole chip portion 114b having a substantially fixed width W1 as the pole width P2W. An outer edge of the yoke portion 114a forms an angle .alpha. with the surface parallel to the air bearing surface 120 at a coupling portion between the yoke portion 114a and the pole chip portion 114b. Moreover, the outer edge of the pole chip portion 114b forms an angle .beta. with the surface parallel to the air bearing surface 120 at the above-mentioned coupling portion. In this case, .alpha. is about 45 degrees, for example, and .beta. is 90 degrees. The width of the pole chip portion 114b defines the write track width on the recording medium. The pole chip portion 114b includes a portion F in front of the TH0 position (close to the air bearing surface 120) and a portion R at the rear of the TH0 position (close to the yoke portion 114a). As can be seen from FIG. 39, the portion F extends on the flat write gap layer 108, and the portion R and the yoke portion 114a extend on a coil portion (hereinafter referred to as an apex portion) which is coated with the photoresist layers 109, 111 and 113 and rises mountainously.
The shape of the top pole is described in Japanese Patent Laid-open No. Hei 8-249614, for example.
The pole width P2W is required to be precisely formed in order to determine the track width of the recording head. More particularly, microfabrication for reducing the pole width P2W of the top pole to 1.0 .mu.m or less in dimension has been recently required in order to enable recording at high surface density, that is, in order to form the recording head having the narrow track structure.
Frame plating method is used as the method of forming the top pole, as disclosed in Japanese Patent Laid-open No. Hei 7-262519, for instance. To form the top pole 114 by using the frame plating, a thin electrode film made of permalloy, for example, is first formed over the apex portion by sputtering, for example. Then, the electrode film is coated with a photoresist, and the photoresist is patterned by photolithography process, whereby a frame for plating is formed. Then, the top pole 114 is formed by the plating by using the previously formed electrode film as a seed layer.
On the other hand, a difference in height between the apex portion and the other portions is 7 .mu.m to 10 .mu.m or more, for example. This apex portion is coated with the photoresist with a thickness of 3 .mu.m to 4 .mu.m. Assuming that the photoresist on the apex portion requires a film thickness of 3 .mu.m or more at the minimum, a photoresist film having a thickness of 8 .mu.m to 10 .mu.m or more, for example, is formed under the apex portion because the photoresist having fluidity collects at the lower place.
In order to form the narrow track as described above, it is necessary to form a frame pattern of about 1.0 .mu.m in width by the photoresist film. That is, a micro pattern of 1.0 .mu.m or less in width must be formed by the photoresist film of 8 .mu.m to 10 .mu.m or more in thickness. However, it is very difficult for a manufacturing process to form such a thick photoresist pattern with a narrow pattern width.
Moreover, during exposure for the photolithography, a light for the exposure is reflected by an underlying electrode film serving as the seed layer. A peripheral region of the photoresist coated with a photomask is exposed to this reflected light. This causes a deformation or the like in the photoresist pattern, and thus a sharp and precise photoresist pattern cannot be obtained. Consequently, the top pole cannot be formed into a desired shape, e.g., the side wall of the top pole is round in shape. For example, when an attempt is made to further reduce the pole width P2W to W1A as shown in FIG. 43 by using a positive photoresist as the photoresist, it is further difficult to obtain this desired width W1A. This is caused for the following reason. In the portion R of the pole chip portion 114b extending on the apex portion, the returned light reflected by the underlying electrode film includes not only the vertically reflected light but also the light reflected obliquely or transversely from an inclined surface of the apex portion. As a result of these reflected lights having an influence upon the exposure of the photoresist layer, a photoresist pattern width for defining the pole width P2W is larger than an intended value. As a consequence, the shape of the pole width P2W becomes the shape shown by a solid line in FIG. 43. In this drawing, a broken line represents the shape of a photomask 130 used for patterning the photoresist.
The width of the portion F of the pole chip portion 114b in front of the TH0 position is a very important factor for defining the track width on the recording medium. Thus, when the width of the portion F is larger than the above-mentioned value W1A, an intended minute track width cannot be obtained.
On the other hand, the improvement in, for example, so-called NLTS (Non-Linear Transition Shift) properties requires minimization of the length of the magnetic path, i.e., the length of the portion to be the path of the magnetic flux generated by the thin film coils. Thus, a sufficient reduction in the throat height TH is required. NLTS expresses, as a percentage, an amount of shift of an actual magnetic recording position from an ideal magnetic recording position on the disk. For example, as shown in FIG. 44, when the amount of polishing is increased during formation of the air bearing surface 120 and thereby the throat height TH is reduced compared to the throat height TH of FIG. 43, a width W1B of the pole chip 114b on the air bearing surface is surely larger than the width W1A of the pole chip 114b of FIG. 43. It is therefore difficult to obtain the intended minute track width.
Such a problem similarly exists in the magnetic head described in Japanese Patent Laid-open No. Hei 8-249614 mentioned above. In the magnetic head described in this publication, the pole width is gradually changed from the TH0 position toward the yoke portion. Thus, the light reflected obliquely or transversely from the inclined surface of the apex portion has a considerable influence upon the exposure of the photoresist layer. Therefore, the width of the portion in front of the TH0 position cannot be precisely controlled.
Moreover, as shown in FIGS. 43 and 44, the portion R of the pole chip portion 114b at the rear of the TH0 position has substantially the same width as the width of the portion F in front of the TH0 position, and thus the portion R has a small cross-sectional area. Thus, the magnetic flux from the yoke portion 114a is saturated in the portion R, and therefore the magnetic flux cannot sufficiently reach to the portion F for defining the track width. Thus, the following problem exists. Overwrite properties, i.e., the properties of overwriting data on the data already written on the recording medium is as low as about 10 dB to 20 dB, for example. Consequently, sufficient overwrite properties cannot be ensured.
For example, as shown in FIGS. 45A and 45B, a so-called stitched pole type thin film magnetic head is also proposed. Specifically, another pole chip portion 118a having the width narrower than the width of the pole chip portion 114b is formed under the pole chip portion 114b that is a part of the top pole 114, and the pole chip portion 118a is magnetically coupled to the pole chip portion 114b. In this drawing, the first-layer thin film coils 110 are located on a thick insulating layer 116 formed on the write gap layer 108, and a magnetic layer 118b formed by the same process as the process of the pole chip portion 118a is located at the rear of the insulating layer 116. According to this thin film magnetic head, the pole chip portion 118a is formed on the flat write gap layer 108, and thus it is relatively easy to reduce the width of the pole chip portion 118a for delimiting the track width on the recording medium. Therefore, the write track width on the recording medium can be reduced. However, also in this type of thin film magnetic head, the photoresist pattern of the portion associated with the formation of the pole chip portion 118a may be spread along the width by the influence of the reflected light from an underlying layer during the exposure. As a result, it is difficult to make the width of the pole chip portion 118a uniform and sufficiently narrow.
The invention is designed to overcome the foregoing problems. It is an object of the invention to provide a method of manufacturing a thin film magnetic head which can precisely control the pole width and can obtain sufficient overwrite properties even when the pole width is reduced.